The present invention relates to a method and apparatus for sealing the passageway between two conduits located in an injection mold and transporting high pressure, heated molten plastic towards mold cavities in a cycling operation.
Avoiding leakage of the hot and pressurized molten plastic material represents a major concern when designing an injection molding machine or an injection mold. For example, a critical leakage area in an injection molding machine is between the machine's injection nozzle and the mold's sprue bushing. Leakage commonly appears in an injection molding machine between the manifold and the mold nozzles or at the interface between the mold nozzle and the cavity mold.
Injection molding manifolds are usually made of a massive runner block communicating with the injection nozzles located adjacent the mold cavities. Leakage of the molten plastic material traveling from the runner block to the injection nozzle, for example, represents a major problem caused by the high pressure of the heated, flowing molten plastic and the relative thermal expansion of the materials which makes the runner block slide laterally with respect to the injection nozzles. Sealing of the fluid passageway between the internal conduits located in the runner block or manifold and the injection nozzle represents, therefore, a crucial design problem, especially taking into account that the injection process must be stopped if leakage occurs.
Several sealing methods and elements have been developed, but these do not satisfactorily solve the leakage problem especially at the interface between the manifold and the mold nozzle.
Known design concepts in injection molds use a small pre-load in cold conditions between the manifold and the nozzle. This additional pre-load accompanies the inherent thermal expansion of the manifold to provide sufficient compression between the parts to maintain sealing between the manifold and nozzle or between other conduits in the system. However, while too little compression results in plastic leakage, extreme compression causes either permanent setting of the manifold steel or damage to the nozzle housing. In addition to this, the prolonged and cyclic injection molding operation will reduce the effectiveness of the pre-load, thus increasing the likelihood of leakage.
Several improvements to these design concepts have been developed that use different methods and means to prevent leakage of the plastic resin.
U.S. Pat. No. 4,588,367 to Schad teaches a thermal expansion element to seal the flow of resin through the passageway between the manifold conduit and injection nozzle, or a thermal expansion element with an undercut to give it additional elastic sealing properties, or a thermal expansion element with a spring element to enhance the sealing properties by adding an elastic feature. The thermal expansion element allows relative movement between the manifold, thermal expansion element and nozzle.
U.S. Pat. No. 5,374,182 to Gessner uses a spring which deflects as the nozzle body and the air piston housing expand due to the increase in temperature. The sealing device uses Belleville style disc springs assembled on an insulating sleeve. As the manifold heats up the disc spring package absorbs the thermal expansion and prevents over-stressing the nozzle housing or setting of the manifold's plate steel. This design provides a superior anti-leaking solution in many situations where the injection pressure remains relatively small. The disc spring system of the '182 patent loads the flange of the nozzle housing in a purely axial direction perpendicular to the interface surface between the nozzle and the manifold plate. By providing an axial sealing force, the profile of the sealing stress shows a significant decrease towards the melt channel relative to the peak achieved at the point of contact between the spring and the nozzle. In the event that the injection pressure reaches higher valves this improved design does not effectively prevent leakage of the molten plastic resin outside the passageway.
U.S. Pat. No. 5,507,637 to Schad et al. teaches a sealing clamp ring attached to the manifold and surrounding the nozzle housing that prevents leakage of the resin at the interface between the manifold and the nozzle. A certain lateral clearance that remains between the clamp ring and the nozzle allows the manifold and the clamp ring to slide laterally without affecting the alignment of the nozzle tip with respect the mold gate.
The design concepts of the '367, '182 and '637 patents represent a significant advance and use a sliding interface between the manifold and the nozzle housing. As the manifold heats and expands it also slides across the nozzle housings which are held in the cavity plate counter bores. This allows the nozzle tip location to be maintained in proper alignment with the mold gate, independent of the temperature of the manifold. However, in a sliding interface between the nozzle and the manifold it is difficult to fully seal the passageway between the conduits of the two parts using the sealing means disclosed in these references and one cannot achieve a sealing stress distribution which has its peak adjacent the passageway.
Accordingly, it is a principal object of the present invention to provide an improved method and apparatus for sealing the passageway between two conduits in an injection molding apparatus, especially between an injection nozzle conduit and a hot runner manifold conduit.
It is a further object of the present invention to provide an improved method and apparatus as aforesaid which obtains a sealing stress distribution which has its peak adjacent the conduits or passageways.
Further objections and advantages of the present invention will appear hereinbelow.